Boston will pause to mourn, and heal, today after the fatal terror attack on the city's beloved marathon.
President Barack Obama, who on Wednesday signed an emergency declaration allowing for expanded federal support for Boston, will be among the speakers at a public interfaith prayer service in the city's Cathedral of the Holy Cross.
Mitt Romney and Michael Dukakis, former U.S. presidential candidates, will be among four ex-Massachusetts governors in attendance at the interfaith service commemorating the victims of the Monday attack at the Boston Marathon, Gov. Deval Patrick's office said.
William Weld and Jane Swift also are scheduled to attend the service at the Cathedral of the Holy Cross in Boston.
Investigators say the bombs, which exploded 12 seconds apart, were designed to deliver the most vicious suffering.
One was housed in a pressure cooker hidden inside a backpack, the FBI said. The device also had fragments that may have included nails, BBs and ball bearings, the agency said.
The second bomb was in a metal container, but it was unclear whether it was in a pressure cooker as well, the FBI said.
Photos obtained by CNN show the remains of a pressure cooker found at the scene, along with a shredded black backpack and what appear to be metal pellets or ball bearings.
They were sent to the FBI's national laboratory in Virginia, where technicians will try to reconstruct the devices.
In the past, the U.S. government has warned federal agencies that terrorists could turn pressure cookers into bombs by packing them with explosives and shrapnel, and detonating them with blasting caps.
While the clues moved the investigation forward, it is still unclear whether the attack was an act of domestic or foreign terrorism.